How to change Login Screen background in Ubuntu 11.10 LightDM

Ubuntu 11.10 Oneiric Ocelot has released its second alpha. In Ubuntu Oneiric, it starts using LightDM as default to instead Gnome Display Manager(GDM). If you have upgraded to or new installed Ubuntu 11.10 Oneiric, you might want to change the LightDM login screen background.